Chemistry: Year In Review 1994

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In an achievement regarded as a milestone in synthetic organic chemistry, two research groups in 1994 announced development of techniques for the total synthesis of the anticancer drug taxol. Originally isolated from the Pacific yew tree, taxol was regarded as a promising treatment for a variety of cancers, including those of the ovary, breast, and lung. At first, obtaining taxol in quantity had been expected to require the cutting and processing of thousands of trees, leading to concern about destruction of yew forests. The shortage in supply set off a worldwide race among organic chemists to obtain the molecule from other sources, yet its total synthesis from simple starting materials proved to be one of the most elusive goals of the past decade. The taxol molecule (seeFigure) is large and complex, built from an unusual system of four rings extremely difficult to re-create in the laboratory.

The two techniques to taxol synthesis are different and were developed by separate research groups. Robert A. Holton and co-workers of Florida State University used ordinary camphor as a starting material and proceeded with a "linear" strategy to assemble each component of the molecule one piece after another. By contrast, K.C. Nicolaou and co-workers of the Scripps Research Institute, La Jolla, Calif., and the University of California at San Diego used a "convergent" strategy in which two large parts of the taxol molecule are synthesized separately and then joined.

Neither synthesis was expected to have an immediate impact on the commercial supply of taxol, which no longer was scarce. Taxol was being made in a semisynthetic process from chemical precursors collected from yew needles and twigs, which can be harvested without killing trees. But scientists said that the work could pave the way for a simpler total synthesis and that it had expanded knowledge about synthesizing complex molecular structures.

Natural gas, best known as a fuel for home heating and cooking, is typically 85-90% methane (CH4). Researchers long have sought cheaper and better ways for exploiting the methane in natural gas as a raw material for making industrial chemicals that currently must be made from petroleum. Doing so has proved difficult because methane does not readily undergo the proper chemical reactions.

During the year Ayusman Sen and Minren Lin of Pennsylvania State University reported developing a single-step process that converts methane into acetic acid (CH3COOH) under mild conditions. In addition to being the acid in vinegar, acetic acid is a key raw material of the chemical industry, used in the manufacture of plastics, pharmaceuticals, pesticides, dyes, and other products. Most industrial acetic acid has been obtained from petroleum. Sen and Lin’s process requires only methane, carbon monoxide (CO), oxygen (O2), and a catalyst, rhodium chloride (RhCl3), which is dissolved in water to promote the conversion of methane. The reaction, which can be summarized as CH4 + CO + 1/2O2 → CH3COOH, gives high yields and produces only methanol and formic acid as by-products. Importantly, the reactions require temperatures of only 100° C (212° F), the boiling point of water. By contrast, a process used for manufacturing acetic acid from methane requires three costly steps, consumes much energy, and requires hazardous organic solvents that must be contained or recycled. The researchers regarded the new process as an important first step toward exploiting the methane in natural gas.

Chemists were devoting increased research attention to molecular self-assembly, a phenomenon in which complex molecules form spontaneously from simple components. Some scientists suggested that life on Earth originated in such a way, with simple chemical components spontaneously growing more complex and developing the ability to replicate. In an advance in the understanding of self-assembly, chemists at the University of Birmingham, England, announced discovery of a molecule that pieces itself together in a previously unrecognized way. J. Fraser Stoddart and David Amabilino synthesized the new molecule, which was dubbed olympiadane because its five interlinked molecular rings resemble the logo of the Olympic Games. Many organic compounds are formed from ringlike arrays of atoms that are attached by chemical bonds between atoms. Olympiadane’s rings, however, are interlocked mechanically without bonds. Stoddart and Amabilino encouraged the self-assembly by careful control of temperature, pressure, and other conditions during synthesis. During assembly, chains of atoms thread together one inside the other, much like the links on a chain, ending with five interlocked rings.

Bleach additives in laundry detergent powders work by oxidizing fabric stains through the action of hydrogen peroxide. Laundry detergents usually contain a perborate compound that forms hydrogen peroxide when the detergent powder comes into contact with water. Hydrogen peroxide, even when aided by detergent additives that lower the water temperature needed for acceptable bleaching activity, does not bleach effectively unless the water temperature is above 40° C (104° F). Many consumers, however, want to do laundry in cooler water in order to conserve energy and avoid damaging modern fabrics. Chemists thus have searched for low-temperature oxidants that bleach in cooler water.

U.S. and Dutch chemists announced discovery of a family of manganese catalysts, derived from 1,4,7-trimethyl-1,4,7-triazacyclononane, that enhance hydrogen peroxide’s bleaching action in cool water. Ronald Hage headed the research, which was carried out at the Unilever Research Laboratories, Vlaardingen, Neth., and Edgewater, N.J. Hage and co-workers reported that the catalysts work with hydrogen peroxide so that it begins bleaching at about 20° C (68° F).

Inorganic Chemistry

Most solid materials expand when heated as their chemical bonds lengthen and their atoms move farther apart. The tendency to expand creates serious problems for solids used in optical, electronic, and other applications. Even slight expansion of materials in telescope mirrors and lasers, for instance, can result in distortion and poor performance. Heat-related expansion is a major cause of premature failure of circuit boards in computers and other electronic devices.

Arthur Sleight and co-workers of Oregon State University announced discovery of a unique family of solid materials that could help solve such problems. The materials--typified by ZrVPO4, an oxide of zirconium (Zr), vanadium (V), and phosphorus (P)--contract steadily when heated between about 200° and 800° C (390° and 1,470° F). Sleight suggested that the unusual behaviour of the materials is due to their crystal structure, in which atoms of vanadium and phosphorus bond not to each other but to an intermediate atom of oxygen. When such a material is heated, the oxygen atom vibrates in a fashion that tends to physically pull the other atoms closer together. The behaviour differs from that of existing materials that resist expansion, such as those used in heat-resistant cookware. Those materials are made of small particles that, when heated, expand in some directions and contract in others, resulting in little net change in volume. But existing materials have disadvantages that limit their use in other applications. Sleight said that compounds such as ZrVPO4 might be used as components in new polymer, graphite, or ceramic composites that would be more versatile yet highly resistant to heat-related failure.

A commission of the International Union for Pure and Applied Chemistry (IUPAC) recommended names for nine chemical elements. The elements, which number 101 through 109 on the periodic table, long had gone without official names because of conflicting claims of discovery and the need for experimental confirmation. The problems were resolved in recent years. All of the elements are unstable and synthetic, having been made in accelerators by fusion of the nuclei of atoms of lighter elements. If approved by the full IUPAC at a meeting scheduled for 1995, the following names and symbols would become part of the periodic table: 101, mendelevium (Md); 102, nobelium (No); 103, lawrencium (Lr); 104, dubnium (Db); 105, joliotium (Jl); 106, rutherfordium (Rf); 107, bohrium (Bh); 108, hahnium (Hn); and 109, meitnerium (Mt). The recommendations caused intense controversy because the commission rejected several names proposed by the discoverers. Scientists who discover a new element traditionally have the right to name it. A stir arose, for instance, over rejection of the name seaborgium (Sg) proposed by the discoverers of element 106. The name would have honoured Nobel laureate Glenn T. Seaborg, the codiscoverer of plutonium and nine other transuranic elements.

In November Peter Armbruster and co-workers at the GSI (Heavy Ion Research Center), Darmstadt, Germany, announced the discovery of element 110. They created three atoms of the element by fusing nuclei of isotopes of nickel and lead in GSI’s heavy-ion accelerator. The following month Armbruster’s team announced that they had made element 111 by fusing nickel and bismuth nuclei.

Biochemistry

An enzyme called ATP synthase is the central energy-generating molecule in almost all forms of life. This protein promotes, or catalyzes, the synthesis of adenosine triphosphate (ATP), which stores chemical energy in a special bond, termed a high-energy phosphate bond. When the bond is broken, or hydrolyzed, thereby separating a phosphate group from the rest of the ATP molecule, the stored energy becomes instantly available. By means of additional chemical reactions, that energy can be transformed into energy needed, for example, to make muscle cells contract, assemble amino acids into proteins, or transmit signals along nerve fibres. In animals ATP is formed in cellular substructures termed mitochondria as nutrients are metabolized. Plants form ATP inside their chloroplasts as photosynthesis converts sunlight into chemical energy. Certain bacteria produce ATP in their cell membranes.

Many biochemists worldwide have studied ATP synthase’s structure and function since it was first isolated in 1960. In an advance heralded as a landmark in those efforts, British biochemists in 1994 reported the deciphering of the atomic structure of a key portion of the ATP synthase molecule. John E. Walker of the Medical Research Council Laboratory of Molecular Biology, Cambridge, headed the research. Walker’s group spent 12 years studying the biochemistry of ATP synthase and trying to grow high-quality crystals of the enzyme. Crystals were necessary for analyzing the enzyme’s structure via X-ray diffraction techniques.

Researchers said the work would help answer many questions about the way living organisms produce energy. Walker also predicted that the structural determination would lead to new insights into the molecular basis of aging. Mitochondrial genes that direct the production of part of the ATP synthase molecule mutate at a much faster rate than conventional genes in a cell’s nucleus. Walker and other scientists suspected that the mutations accumulate with time as an organism ages. The changes impair an organism’s ability to produce energy and may be a key factor in Parkinson’s disease, Alzheimer’s disease, and other degenerative diseases of aging. Michael Woods